A site that has proven particularly sensitive to antibody blockade of lymphocyte infection falls in the third variable domain (V3) loop of the HIV envelope glycoprotein (gp) 120. The research program proposed here is directed solely at this region of the HIV envelope in an effort to understand its structure and function and how the observed amino acid sequence variation within this domain influences infectivity as well as antibody reactivity. The work will be extended to infection of primary monocytes/macrophages in addition to T cell lines since details of attachment, penetration, and role of antibody in cells of these two lineages may differ. The major questions addressed and approaches followed are four fold. First, the investigator proposes to test if antibodies to V3 neutralize, enhance, or have no effect on infection of primary monocytes/macrophages. Second, the structure/function of the V3 domain is unknown but interaction with a membrane protease has been proposed and this possibility will be tested. Third, site-directed mutagenesis of the HXB2 cloned isolate will be utilized to identify sequence and structural features within this variable domain most critical for virus infectivity. The steps at which the non-infectious virus constructs are blocked will be characterized using a simian virus 40 (SV40)-based vector to express and study the mutant envelope glycoprotein in the absence of other HIV components. Finally, mutagenized HIV prepared in the third aim above which are replication competent will be studied in neutralization assays to define residue sites which have the greatest impact on neutralizing antibody.